@jamieman5 

WiFi mesh is a group of WiFi range extenders that are working together to provide seamless WiFi network coverage around home.

To understand how WiFi mesh works it is necessary to learn how WiFi range extenders work. There is plenty of info on that topic if you Google it, but I have found for you an easy to digest article that I would suggest you read first. 

It will have the answer to your question under the section "How do Wi-Fi extenders work?" That answer applies to WiFi mesh, too.

Article: Wi-Fi extender 101: What it is and why you might need it

Author: Samantha Cossick

URL: https://www.allconnect.com/blog/3-reasons-house-needs-wi-fi-extender

In the middle of that article you'll find phrase "Another option is a mesh WiFi network that uses multiple, smaller routers to create a network of wireless signals around your home" with the link to the article about WiFi mesh systems.

I would recommend reading "WiFi Extender 101" article till the end, and then follow the link to read about mesh WiFi, now that you have general understanding of WiFi range extenders.

If after reading these two articles you have questions, specific to Deco mesh, feel free to ask them here.

If you find you need more detailed overview of general principles of WiFi routers and WiFi mesh technology, perhaps Google can help better.